Technical Field
The present invention relates to an encoder, and particularly, to an optical encoder.
Related Art
There are now optical linear encoders as one type of devices for measuring an amount of movement. An optical linear encoder has a scale, and a detection head configured to move along the scale. The scale has an absolute pattern for detecting a reference position, and an incremental pattern for detecting an amount of relative movement between the scale and the detection head. The optical linear encoder determines a reference position based on a reference position signal representing a result of detection on the absolute pattern formed on the scale. Then, the optical linear encoder can detect the positional relation between the scale and the detection head, in view of an amount of movement from the reference position.
For example, as an example of such an optical encoder, there has been proposed a reference position detection device configured to make a dark area by a phase difference in light at a reference position and detect the dark area, thereby detecting a reference position (Patent Document 1). In this reference position detection device, a main scale allows parallel beams to pass in an arbitrary light-and-dark pattern. At this time, steps provided on the main scale cause a difference in light path length between an area where there are steps and an area where there are no steps. Therefore, between beams having passed through the area where there are the steps and beams having passed through the area where there are no steps, a phase difference occurs. For this reason, at the boundary between the area where there are the steps and the area where there are no steps, beams passing through the two areas interfere with each other, resulting in a reduction in light intensity, whereby a dark area occurs. A light receiving device measures the amount of beams having passed through a reference position slit. While the main scale moves, the amount of beams which the light receiving device detects becomes smallest when the dark area passes over the reference position slit. Therefore, it is possible to detect that position as the reference position.
Patent Document 1: Japanese Patent Application Laid-Open No. 10-2717
The above-described reference position detection device has a problem that the accuracy of reference position detection decreases due to change of temperature. If the temperature of the above-described main scale rises, the step height changes, and the light path length according to the steps also changes. For this reason, the phase difference between beams having passed through the area where there are the steps and beams having passed through the area where there are no steps also varies, and the contrast of the dark area decreases. As a result, a change in the accuracy of detection of the reference position is caused.